Alkaline Earth Metal Cation Exchange: Effect of Mobile Counterion

Mar 16, 2012 - Department of Environmental Engineering Sciences, University of Florida, PO Box ... Na, Mg, Ca, Sr, and Ba mobile counterion forms as a...
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Alkaline Earth Metal Cation Exchange: Effect of Mobile Counterion and Dissolved Organic Matter Katrina Indarawis and Treavor H. Boyer* Department of Environmental Engineering Sciences, University of Florida, PO Box 116450, Gainesville, Florida 32611-6450, United States S Supporting Information *

ABSTRACT: The goal of this research was to provide an improved understanding of the interactions between alkaline earth metals and DOM under conditions that are encountered during drinking water treatment with particular focus on cation exchange. Both magnetically enhanced and nonmagnetic cation exchange resins were converted to Na, Mg, Ca, Sr, and Ba mobile counterion forms as a novel approach to investigate the exchange behavior between the cations and the interactions between the cations and DOM. The results show that cation exchange is a robust process for removal of Ca2+ and Mg2+ considering competition with cations on the resin surface and presence of DOM. DOM was actively involved during the cation exchange process through complexation, adsorption, and coprecipitation reactions. In addition to advancing the understanding of ion exchange processes for water treatment, the results of this work are applicable to membrane pretreatment to minimize fouling, treatment of membrane concentrate, and precipitative softening.

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INTRODUCTION Increasing population, urbanization, and climate change represent global game changers concerning the future quantity and quality of drinking water supplies.1,2 Moreover, anthropogenic activities and climate change are expected to increase the concentrations of dissolved organic matter (DOM) and total dissolved solids (TDS) in existing freshwater supplies.3,4 As such, many countries will have to augment their existing drinking water supplies with alternative water sources that are characterized by elevated concentrations of DOM or TDS or both. The use of alternative water sources, such as organic-rich surface water or brackish groundwater, is expected to necessitate advanced water treatment trains in which membrane processes will be an integral part of DOM and TDS removal. Membrane fouling, product water recovery, and management of membrane concentrate are considered major challenges to efficient and effective membrane separation.5−7 Water sources that contain both DOM and divalent metal cations (i.e., subfraction of TDS) are known to be problematic in terms of membrane fouling. For instance, Ca2+ can promote the aggregation of DOM molecules and act as a bridge between DOM molecules and the membrane surface, both of which contribute to a dense fouling layer.8−12 Precipitation of CaCO3, CaSO4, BaSO4, and other sparingly soluble minerals can foul membranes and decrease membrane performance.13,14 As a result, various pretreatment processes that target either organic or inorganic foulants have been studied with the goal of reducing membrane fouling.15−17 There is no consensus, however, on the efficacy of pretreatment processes to reduce membrane fouling. One reason for the inconclusive results is that pretreatment processes focus on either DOM removal or © 2012 American Chemical Society

divalent metal cation removal. For example, both anion exchange and cation exchange have been tested as separate processes for DOM removal and divalent metal cation removal, respectively.18−22 However, there is limited research investigating combined anion and cation exchange for simultaneous removal of DOM and divalent metal cations during drinking water treatment.23 The proposed combined ion exchange process uses anion exchange resin and cation exchange resin in a single completely mixed flow reactor or fluidized bed reactor.24 A majority of the resin is continuously recycled within the reactor while a small fraction of resin is regenerated and returned to the reactor. Anion exchange resin and cation exchange resin exhausted with contaminants are regenerated in a single reactor using a concentrated salt solution. The underlying removal mechanism is stoichiometric exchange between aqueous DOM and chloride on the anion exchange resin and metal cations in solution and sodium on the cation exchange resin. Apell and Boyer tested this novel combined anion and cation exchange process at the laboratory scale and showed 70% dissolved organic carbon (DOC) removal and >55% total hardness removal.23 The combined ion exchange process is expected to substantially reduce membrane fouling because both organic and inorganic precursors of membrane fouling are removed. Although the combined ion exchange process was shown to be effective for simultaneous removal of DOC and total Received: Revised: Accepted: Published: 4591

November 30, 2011 March 12, 2012 March 16, 2012 March 16, 2012 dx.doi.org/10.1021/es204289c | Environ. Sci. Technol. 2012, 46, 4591−4598

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Table 1. Composition of Synthetic Model Waters Used in Cation Exchange Experiments model water (metal− DOM) Ca−SRDOM Ca−StMRDOM Ca−no DOM Ca/Mg−SRDOM Ca/Mg−StMRDOM Ca/Mg−no DOM Mg−SRDOM Mg−StMRDOM Mg−no DOM Na−SRDOM Na−StMRDOM Na−no DOM

pH

Na+ (mmol/L)

Mg2+ (mmol/L)

Ca2+ (mmol/L)

Cl− (mmol/L)

SO42− (mmol/L)

DIC (mg/L)

DOC (mg/L)

DOC (meq/Lg)

8.3a 8.5b 8.1c 8.2d nme 8.0g 8.5a 8.5b 8.1c 8.1d nm 8.4g 8.3a 8.6b 8.2c 8.3d nm 8.4g 8.3a 8.7b 8.4c nm 8.6g

3.4a 3.3b 3.8c 3.4d 3.4f 3.4g 3.4a 3.4b 3.8c 3.4d 3.4f 3.4g 3.4a 3.3b 3.8c 3.4d 3.4f 3.4g 3.4a 3.3b 3.8c 3.4f 3.5g

− − − − − − 1.2a 1.3b 1.4c 1.2d 1.2f 1.2g 2.5a 2.6b 2.8c 2.5d 2.4f 2.5g − − − − −

2.4a 2.6b 2.8c 2.5d 2.0f 2.1g 1.2a 1.4b 1.4c 1.3d 1.0f 1.2g − − − − − − − − − − −

4.6a 4.8b 4.6c 4.8d 4.8f 4.2g 4.7a 5.0b 4.7c 4.8d 4.8f 4.2g 4.7a 4.8b 4.7c 4.8d 4.8f 4.3g − − − − −

0.44a 0.45b 0.44c 0.44d 0.44f 0.44g 0.44a 0.46b 0.44c 0.44d 0.44f 0.44g 0.44a 0.45b 0.44c 0.44d 0.45f 0.45g 0.44a 0.45b 0.44c 0.45f 0.44g

27a 28b 28c 27d 19f 20g 28a 28b 28c 27d 23f 28g 28a 28b 23c 27d 28f 29g 28a 29b 28c 29f 29g

5.0a 5.0b 5.0c 5.1d − − 5.5a 5.1b 4.9c 5.5d − − 5.3a 5.1b 5.1c 5.6d − − 5.5a 5.2b 5.2c − −

0.050a 0.051b 0.055c 0.056d − − 0.056a 0.053b 0.054c 0.060d − − 0.053a 0.052b 0.056c 0.063d − − 0.055a 0.054b 0.058c − −

a

MIEX-Na, -Ca, -Mg, -Sr, and -Ba resins with Ca, Ca/Mg, Mg, and Na model waters. bNon-MIEX-Na, -Ca, -Mg, -Sr, and -Ba resins with Ca, Ca/Mg, Mg, and Na model waters. cMIEX-Na resin with Ca, Ca/Mg, and Mg model waters and MIEX-Ca and -Mg resins with Na model water. dMIEX-Ca and -Mg resins with Ca, Ca/Mg, Mg, and Na model waters. eNot measured (nm). fMIEX-Na resin with Ca, Ca/Mg, Mg, and Na model waters. g MIEX-Ca, -Mg, -Sr, and -Ba resins with Ca, Ca/Mg, Mg, and Na model waters. hDOC (g C/L) × change density (meq/g C) at pH of model water; see Figure S1 in SI.

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hardness from a real groundwater,23 the interactions between divalent metal cations, inorganic anions, DOM, and anion and cation exchange resins were not systematically evaluated. In particular, the real groundwater was dominated by Ca2+ and it is not known how competition with Mg2+ or trace concentrations of Sr2+ or Ba2+ would affect the cation exchange process. Interactions between Ca2+ and DOM, and to a lesser extent Mg2+ and DOM, have been studied.25−27 There are fewer studies that have investigated Sr2+−DOM and Ba2+−DOM interactions.28 There are also few previous studies that have investigated the effects of physical−chemical processes on the alkaline earth metal series.29 Finally, although there have been advances in DOM removal by anion exchange,30−33 it is not well-documented in the literature how the uptake of divalent metal cations during cation exchange would perform in the presence of DOM. Thus, the gaps in knowledge surrounding combined ion exchange as a new drinking water treatment process motivated this research to evaluate the fundamental interactions that take place during cation exchange. The goal of this research was to provide an improved understanding of the interactions between alkaline earth metals and DOM under conditions that are encountered during drinking water treatment with particular focus on cation exchange. The specific objectives of this research were to (i) quantify the stoichiometry and selective removal of alkaline earth metals during cation exchange; (ii) evaluate the potential for coremoval of DOM during cation exchange; (iii) evaluate the role of precipitation and complex formation reactions during cation exchange; and (iv) discuss the implications of the results for drinking water treatment.

MATERIALS AND METHODS

Model Waters. Table 1 shows the measured composition of all model waters used in this research. The rationale for the composition of the model waters was to approximate a real groundwater that was previously studied using combined anion and cation exchange,23 and in addition, to vary the speciation of Ca and Mg to represent different sources of hardness in drinking water. The total concentration of divalent metal cations was constant at ∼2.5 mmol/L (∼250 mg/L as CaCO3), while the speciation of divalent metal cations was varied as follows: Ca only, Ca and Mg, Mg only, and zero divalent metal cations (i.e., Na only). The model waters were prepared by dissolving salts and DOM in deionized (DI) water with no additional pH adjustment. The salts included ACS reagent grade purity CaCl2·2H2O, MgCl2·6H2O, NaHCO3, and Na2SO4. The DOM isolates were Suwannee River DOM (SRDOM), International Humic Substances Society 1R101N, and Saint Marys River DOM (StMRDOM) isolated from the Saint Marys River in Florida.34 The charge density of the DOM isolates was obtained by potentiometric titration as described in Supporting Information (SI) and shown in Figure S1. Visual MINTEQ was used to evaluate the stability of the model waters as discussed in SI and shown in Tables S1, S2, and S3. The model waters were limited to inorganic cations (Na+, Mg2+, Ca2+, Sr2+, Ba2+), inorganic anions (Cl−, SO42−, HCO3−/ CO32−), and DOM because the focus of the work was cation exchange. Although the results of this work have implications for membrane fouling, the model waters did not include all possible foulants such as silica. 4592

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Figure 1. Cation exchange stoichiometry for Na-, Mg-, Ca-, Sr-, and Ba-forms of MIEX resin. Legend indicates metal−DOM composition of model water.

Cation Exchange Resin Preparation. Two cation exchange resins were used in this work: MIEX WA172 (hereafter MIEX) and Purolite C106Na (hereafter nonMIEX). Both resins had the same pore structure, polymer composition, and weak-acid functional groups as shown in Table S4 in SI. MIEX resin contained magnetic iron oxide giving it magnetic properties that were not present in nonMIEX resin. The resins were converted to the following mobile counterion forms: sodium (R-Na), magnesium (R2-Mg), calcium (R2-Ca), strontium (R2-Sr), and barium (R2-Ba) where R represents either MIEX or non-MIEX resin and the subscript 2 shows that divalent cations occupy two exchange sites on the resin. MIEX and non-MIEX resin were converted to the appropriate mobile counterion by regenerating the resins in a concentrated metal chloride salt solution that contained 25× stoichiometric excess of the target mobile counterion, which was calculated based on the ion exchange capacity of the resins as reported by the manufacturers. The regeneration procedure has been used previously by the authors to convert ion-exchange resin to different mobile counterion forms and regenerate exhausted ion-exchange resin.23,35,36 Cation Exchange Experiments. Batch equilibrium experiments were conducted by adding 1.6 mL of MIEX resin or 0.31 mL of non-MIEX resin to 100 mL of model water. The MIEX resin dose was based on DOC removal in previous work.23 The non-MIEX resin dose was calculated so that both MIEX resin and non-MIEX resin were tested at the same meq/L dose based on the ion exchange capacity of the resins. Accurate and precise dosing of ion-exchange resin on a volumetric basis is challenging for small volumes of resin. As a result, MIEX resin and non-MIEX resin were filtered, dried in a desiccator, weighed on an analytical balance, and added as dry resin based on previous work.30,31 Samples were mixed for 2 d on a shaker table at 200 rpm to ensure full equilibrium. The equilibrium time was determined from preliminary kinetic experiments. In practice, ion exchange is a rapid process in which effective treatment is achieved in 60 min or less.24,37 All combinations of cation exchange resin and model water were tested in duplicate. All results show the mean of duplicate samples with error bars showing the standard error as described in SI. All samples were

filtered through 0.45-μm nylon membrane filters after 2 d of mixing. The filters were prepared by rinsing with 500 mL of DI water and ∼10 mL of sample. DOC and dissolved inorganic carbon (DIC) were measured by high-temperature combustion on a total organic carbon analyzer (Shimadzu, TOC-VCPH). Ultraviolet absorbance was measured on a UV−visible spectrophotometer (Hitachi, U-2900) at 254 nm (UVA254) using a 1-cm quartz cuvette. Inorganic cations (Na+, Mg2+, Ca2+, Sr2+, Ba2+) and anions (Cl−, SO42−) were measured by ion chromatography with conductivity detection (Dionex, ICS3000). Calibration standards were prepared from appropriate ACS reagent grade purity salts. Operating conditions for ion chromatography are described in SI. An Acumet AP71 pH meter with a pH/ATC probe was used to measure pH.

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RESULTS AND DISCUSSION

Stoichiometry of Cation Exchange. Ion exchange differs from ordinary adsorption processes in that there is a charge balance during the exchange process in which the total concentration of ions removed from solution is equal to the total concentration of ions released from the resin. The cation exchange stoichiometry of MIEX resin for different mobile counterions and under different water chemistry conditions is shown in Figure 1. Cation removal and cation release are expressed in meq/L to account for exchange of monovalent and divalent cations. The y = x line represents the stoichiometric exchange of cations in solution with cations on the resin. Data that fall along the y = x line indicate ideal cation exchange behavior. The results in Figure 1 illustrate the following key points. Aqueous Ca2+, in both the absence and presence of SRDOM, exchanged stoichiometrically with resin-phase Na, Mg, and Sr. Similarly, aqueous Mg2+, in both the absence and presence of SRDOM, exchanged stoichiometrically with resin-phase Na, Ca, and Sr. The negligible effect of SRDOM on cation exchange stoichiometry was because the meq/L concentration of divalent metal cations was approximately 50× higher than the meq/L concentration of SRDOM (see Table 1). All data for MIEX-Ba resin were systematically shifted to the left of the y = x line indicating more cations removed from solution than 4593

dx.doi.org/10.1021/es204289c | Environ. Sci. Technol. 2012, 46, 4591−4598

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Figure 2. Calcium and magnesium removal for Na-, Mg-, Ca-, Sr-, and Ba-forms of resin in the absence/presence of SRDOM: (a−c) Ca only or Mg only in aqueous phase (∼2.5 mmol/L) and (d−f) both Ca (∼1.25 mmol/L) and Mg (∼1.25 mmol/L) in aqueous phase. Error bars show the standard error for duplicate samples. Magnesium release from Mg-form resin and calcium release from Ca-form resin not shown.

from solution (i.e., cation exchange resin = 8.3 meq/L, divalent metal cations ≈5 meq/L). MIEX resin showed a similar affinity for Ca2+ and Mg2+ when there was no competition between Ca2+ and Mg2+ for ion exchange sites and there was no DOM in solution (Figure 2a). Competition between Ca2+ and Mg2+, and to a lesser extent the presence of SRDOM, resulted in MIEX resin having a greater affinity for Ca2+ over Mg2+ (Figure 2b, d, e). The exception was MIEX-Na resin which showed greater affinity for Mg2+ over Ca2+. The different affinity of MIEX-Na resin relative to Mg-, Ca-, Sr-, and Ba-forms of MIEX resin illustrates the need to understand ion exchange behavior for all relevant mobile counterions. Figure S4 in SI shows the selective removal of Ca2+ and Mg2+ by MIEX resin in the presence of StMRDOM; the results follow similar trends as MIEX resin in the presence of SRDOM with MIEX-Na resin having a different affinity than the Mg- and Ca-forms of MIEX resin. Mg-, Ca-, Sr-, and Ba-forms of non-MIEX resin showed a greater affinity for Ca2+ over Mg2+ in the presence of SRDOM without and with competition between Ca2+ and Mg2+. The Naform of non-MIEX resin showed nearly complete removal of Mg2+ and Ca2+ so it was not possible to determine an affinity. All mobile counterion forms of non-MIEX resin showed greater removal capacity for Ca2+ and Mg2+ than the corresponding form of MIEX resin. The higher capacity of non-MIEX resin was most evident when comparing MIEX-Na and non-MIEXNa resins in which non-MIEX-Na resin achieved essentially complete removal of Ca2+ and Mg2+, which is reasonable considering the meq/L resin dose was greater than the divalent cation concentration. The doses of the two cation exchange resins were estimated at approximately the same meq/L dose following data from the manufacturers; however, the results suggest that the meq/L dose of non-MIEX resin was greater than that of the MIEX resin. Non-MIEX resin is commercially available so there is more certainty in its ion exchange capacity whereas the cationic form of MIEX resin is not commercially available. As a result, MIEX resin may have a lower capacity than provided by the manufacturer. DOM Co-Removal during Cation Exchange. Many types of DOM contain carboxylic acids that are deprotonated at pH 8, −COO−, giving DOM a net negative charge.39 As a result,

released by the resin. This typically suggests adsorption, but is most likely indicative of Ba mineral precipitation as discussed in a later section. As expected, there was minimal exchange between aqueous Na+ and the resin-phase alkaline earth metals because of the higher selectivity of MIEX resin for divalent cations; as a control there was no change in the solution composition when the aqueous solute and resin-phase counterion were the same species. Additional cation exchange stoichiometry data for MIEX resin in the presence of StMRDOM and non-MIEX resin in the presence of SRDOM are shown in Figures S2 and S3 in SI. The results follow trends similar to those in Figure 1. For instance, ideal cation exchange behavior for R-Na, R2-Mg, R2-Ca, and R2-Sr, and more cations removed from solution than released by non-MIEX-Ba resin suggesting Ba mineral precipitation. Selective Removal during Cation Exchange. The selective removal of Ca2+, Mg2+, and other metals during cation exchange is well-established using Na as the mobile counterion.38 However cation exchange treatment is a dynamic process in which the dominant exchange reaction can be between two or more solutes after replacing the mobile counterion. For example, consider cation exchange treatment of groundwater for hardness removal. Mg2+, Ca2+, and possibly Sr2+ and Ba2+ will first displace resin-phase Na because the resin has higher selectivity for divalent cations than monovalent cations. Then aqueous divalent cations will exchange with resinphase divalent cations because of differences in concentration and selectivity until equilibrium is established. Hence, there is a need to understand the selectivity of cation exchange reactions for which Na is not the mobile counterion. Figure 2 compares the selective removal of Ca2+ and Mg2+ during cation exchange for different resin properties and water chemistry. In Figure 2, a−c correspond to model waters that contain either Ca2+ or Mg2+ (total concentration ∼2.5 mmol/ L), whereas d−f correspond to model waters that contain both Ca2+ and Mg2+ (total concentration ∼2.5 mmol/L). The distinction is important because it allows the selective removal of Ca2+ and Mg2+ to be quantified without and with direct competition. The results in Figure 2 are mostly discussed in terms of affinity (i.e., selective removal) because there was sufficient ion exchange capacity to remove all divalent cations 4594

dx.doi.org/10.1021/es204289c | Environ. Sci. Technol. 2012, 46, 4591−4598

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Figure 3. Change in DOC and UVA254 during cation exchange uptake of metal cations by Na-, Mg-, Ca-, Sr-, and Ba-forms of resin: (a) MIEX resin/ SRDOM, (b) MIEX resin/StMRDOM, and (c) non-MIEX resin/SRDOM. Legend indicates cation composition of model water. Error bars show the standard error for duplicate samples.

(SUVA254), which is a widely used surrogate for aromaticity.42 The SUVA254 values for StMRDOM and SRDOM were approximately 6 and 4 L/mg·m, respectively, indicating that StMRDOM was more enriched in aromatic carbon than SRDOM. Thus, the results for StMRDOM follow from the favorable interactions between aromatic domains of StMRDOM with the resin matrix, Ca2+, and other divalent metal cations. Unlike the results for MIEX resin, there was negligible removal of DOC and UVA254 by non-MIEX resin as shown in Figure 3c (with the exception of non-MIEX-Ba resin indicative of Ba mineral precipitation). Together, the results for MIEX resin and non-MIEX resin indicate that cation exchange uptake of positively charged metal−DOM complexes or surface complexation between DOM and mobile counterions are not the likely removal mechanisms. Both MIEX resin and nonMIEX resin had similar resin properties (Table S4 in SI). The key difference between the resins was the incorporation of magnetic iron oxide into the polymer matrix of MIEX resin. The magnetic iron oxide allows MIEX resin to have a smaller particle size than traditional ion exchange resins and allows for rapid settling and separation of MIEX resin from solution.43 There is no previous research examining the role of iron oxide in MIEX resin on DOM removal. Analysis of MIEX resin by vibrating sample magnetometer showed that the iron oxide was synthetic, single domain magnetite with a particle size less than ∼200 nm (SI, Table S5, and Figure S5). Magnetite has been used previously to prepare magnetic polymer resins and adsorbents.44,45 Previous scanning electron microscope images of anionic MIEX resin showed that the resin surface was highly porous and heterogeneous,35 which suggests that magnetite nanoparticles could be exposed on the resin surface. Thus, the working hypothesis for DOM coremoval during magnetically enhanced cation exchange is adsorption of DOM and metal− DOM complexes to exposed magnetite nanoparticles. The average point of zero charge for synthetic magnetite is 6.8,46 so at the pH of the model waters, pH 8, the magnetite surface is negatively charged, i.e., Fe−O−. The enhanced adsorption of DOM by MIEX resin in the presence of divalent metal cations

DOM is effectively and stoichiometrically removed during anion exchange processes (i.e., exchange between −COO− and resin-phase Cl−),31 and there should be no DOM removal during cation exchange processes (i.e., no exchange between −COO− and resin-phase Na+). However substantial removal of DOC and UVA254 was observed during magnetically enhanced cation exchange (i.e., MIEX-Na resin) in previous work23 and in this study as shown in Figure 3. The baseline condition for the comparison of all results is MIEX resin, Na mobile counterion, zero divalent metal cations, and SRDOM, which resulted in ∼20% DOC removal and ∼30% UVA254 removal (see Figure 3a). Removal of DOC increased for the following conditions: alkaline earth metal used as mobile counterion < model water contained divalent metal cations. For example, DOC removal was 33−47% (MIEX-Mg, -Ca, -Sr, and -Ba resins and Na model water) < 46−53% (MIEX-Na resin and Ca, Ca/Mg, Mg model waters) < 46−67% (MIEX-Mg, -Ca, -Sr, and -Ba resins and Ca, Ca/Mg, Mg model waters). Although the presence of Ca2+ and Mg2+ increased DOC removal relative to Na+, the identity of the divalent metal cation did not significantly affect DOC removal. UVA254 followed the same trend as DOC but showed a greater decrease for all conditions. Aromatic-rich (i.e., UV-absorbing) domains of DOM are known to interact favorably with polymer resins and Ca2+,40,41 which accounts for the greater reduction in UVA254. Greater removals of DOC and UVA254 were measured when Ba was used as the mobile counterion indicative of Ba mineral precipitation. The potential for coremoval of StMRDOM during cation exchange was evaluated for all model waters but restricted to Na, Mg, and Ca mobile counterions because of a limited amount of DOM isolate. The results for StMRDOM follow the same trends as SRDOM in terms of the effects of water chemistry and resin properties on DOC and UVA254 removal (see Figure 3b). The key difference in cation exchange behavior between the DOM isolates was greater removal of StMRDOM than SRDOM. Both DOM isolates had similar acidity as illustrated by their potentiometric titrations (Figure S1 in SI). The DOM isolates, however, differed in their specific UVA254 4595

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in solution suggests that metal−DOM complexes, e.g., Ca2+− DOM, are being removed through electrostatic interactions with negatively charged magnetite. Electrostatic removal of metal−DOM complexes is consistent with previous work showing electrostatic removal of 90Sr by magnetite at alkaline pH.47 The low DOM removal by MIEX resin in the absence of divalent metal cations is consistent with the limited adsorption of DOM by magnetite at alkaline pH.48 Understanding the role of iron oxide in MIEX resin is important because magnetically enhanced polymer adsorbents are increasingly being considered for water treatment, separation processes, and drug delivery.49,50 Precipitation of Barium Minerals. One of the most striking results from Figure 3 was the substantial removal of DOM in experiments in which Ba was used as the mobile counterion and the model water contained divalent metal cations. For example, DOC removal was 61−67% for both MIEX-Ba and non-MIEX-Ba resins in the presence of Ca2+ and/or Mg2+ (Figure 3a and c), which was also the maximum DOC removal observed for all test conditions using SRDOM. There was much less DOC removal by MIEX-Ba resin and there was no DOC removal by non-MIEX-Ba resin when divalent metal cations were absent from the model water. Consequently, the working mechanism of DOM coremoval during Ba-form cation exchange is hypothesized to include exchange of aqueous divalent metal cations with resin-phase Ba, release of Ba into bulk solution, precipitation of Ba minerals, DOM coprecipitation with or adsorption to Ba minerals, and DOM removal during solid−liquid separation. The precipitation of Ba minerals during Ba-form cation exchange is supported by the sulfate data shown in Figure 4.

summarized in Tables S2 and S3 in SI. Exchange of Ca2+ and Mg2+ with R2-Ba resin resulted in release of Ba2+ and supersaturated conditions for Barite (BaSO4) and witherite (BaCO3). Hence, all results support the working hypothesis of Ba mineral precipitation during Ba-form cation exchange. Final confirmation of the hypothesis would require collection and analysis of the precipitate, which was not conducted. Although precipitation of Ba minerals on membrane surfaces is a known problem,7 precipitation of Ba minerals on ionexchange resin, and subsequent fouling of the resin, is not expected to occur because of separation of anions and cations on respective resins. For instance, during the combined ion exchange process sulfate is loaded on the anion exchange resin while barium is loaded on the cation exchange resin. The precipitation of BaSO4 would not occur until the regeneration process in the bulk solution and not on the resin. Ion exchange waste brine and membrane concentrate are high ionic strength solutions favorable for Ba mineral precipitation, whereas natural waters are less favorable for Ba mineral precipitation (see SI). Implications for Drinking Water Treatment. The motivation for this research was a new combined anion and cation process that can simultaneously treat DOM and metal cations.23 The alkaline earth metals were the target for cation exchange because Ca and Mg are the dominant sources of hardness in drinking water, and Mg, Ca, Sr, and Ba form sparingly soluble carbonate and sulfate minerals that can foul nanofiltration and reverse osmosis membranes thereby affecting contaminant rejection and water recovery. The cation exchange resins were tested using Na, Mg, Ca, Sr, and Ba as mobile counterions to understand the complex reactions that can occur during combined ion exchange treatment. The results of this work are expected to advance two main directions for combined ion exchange: as an alternative to conventional multiprocess DOM and hardness removal (e.g., coagulation followed by lime softening) and as a pretreatment process for membranes that removes both inorganic and organic precursors to membrane fouling. Considering combined ion exchange as an alternative to multiprocess DOM and hardness removal, the presence of DOM did not adversely affect the removal of Ca2+ or Mg2+ by cation exchange. In contrast, precipitative softening is influenced by DOM with softening performance decreasing as DOM concentration increases.41,51 Considering combined ion exchange as a pretreatment for membranes, precipitation of sparingly soluble minerals will occur during the regeneration process and not on the resins or membrane surface thus minimizing fouling. This work can be reasonably extended to combined ion exchange treatment of terrestrially derived DOM from organic matter impacted aquifers and surface waters. The model waters tested had a DOC concentration of 5 mg/L, which is high for groundwater although not unrealistic for aquifers with organicrich sediment or impacted by surface water.52 In addition, many surface waters used as drinking water sources have DOC concentrations of ∼5 mg/L. SRDOM and StMRDOM are representative of terrestrially derived DOM found in surface water. Microbially derived DOM, such as wastewater effluent organic matter, was not investigated. Key differences between microbially and terrestrially derived DOM include aromatic carbon content and carboxyl acidity with microbially derived DOM lower in both.53,54 Aromatic hydrocarbons and carboxylic acids in terrestrially derived DOM lead to favorable interactions with divalent metal cations, mineral surfaces, and ion exchange resins.40,41 Thus additional work is needed to

Figure 4. Change in dissolved sulfate during cation exchange uptake of metal cations by Ba-form of MIEX and non-MIEX resins in the absence/presence of SRDOM. Legend indicates cation composition of model water. Error bars show the standard error for duplicate samples.

The sulfate concentration was decreased by >90% during Baform cation exchange in the presence of Ca2+ or Mg2+. In the absence of divalent metal cations (i.e., Na+ only), there was much less sulfate removal by MIEX-Ba resin and there was no sulfate removal by non-MIEX-Ba resin. There was no change in the sulfate concentration for Na-, Mg-, Ca-, and Sr-form cation exchange for all model waters except MIEX-Sr resin in the presence of Ca2+ (Figure S6 in SI), and there was no change in the chloride concentration for all cation exchange experiments (Figure S7 in SI) thus ruling out anion exchange. Finally, Visual MINTEQ was used to model the precipitation potential of the various model waters before and after cation exchange as 4596

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understand the applicability of the results to microbially derived DOM, such as membrane pretreatment for wastewater applications. This work is specific to cation exchange resin with macroporous structure, polyacrylic composition, and carboxylic acid functional groups. Alternative cation exchange resin properties include gel structure, polystyrene composition, and sulfonic acid functional groups.37,38 Cation exchange reactions between Na, Mg, Ca, Sr, and Ba are not expected to be affected by these changes in resin properties. However, DOM uptake by anion exchange resin is affected by pore structure and polymer composition. For instance, gel resins can limit DOM uptake through size exclusion and polystyrene resins show favorable intermolecular attractions with DOM.40,55 Thus, this work is expected to be applicable to other cation exchange resin properties with the caveat that gel- or polystyrene-type cation exchange resins could exhibit size exclusion or enhanced interactions or both with metal−DOM complexes.

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ASSOCIATED CONTENT

* Supporting Information S

Additional methods description and figures/tables. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 352-846-3351; fax: 352-392-3076; e-mail: thboyer@ufl. edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.I. is supported by a UF Graduate Alumni Fellowship. T.H.B. received a gift of $11,000 from Orica Watercare in August 2010. We thank Purolite and Orica Watercare for providing cation exchange resins. We also thank Dr. Andrew Zimmerman and Dr. James Channell, Department of Geological Sciences, UF, for their guidance and analysis of the magnetic properties of MIEX resin. This paper was improved by the thoughtful comments of three anonymous reviewers.

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REFERENCES

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